Various microfiche readers are known. They usually comprise a light source, image enlarging and projecting optics aligned with light source and a film holder therebetween. A mechanical or electro mechanical transport mechanism is also provided for moving the film holder rectilinearly and indexing the film.
A glass plate is placed between the film and the optics, protects the film from contact and/or damage by the optics and securely clamps the film to the holder. It also provides the means for controlling the spacing between optics and the film surface.
The holder moving mechanism is relatively complicated and expensive to construct since the film holding table must be linearly movable in two perpendicular directions. Additionally, the film clamping and protecting glass plate prevents an accurate gauging of film surface irregularities so that the film optics is positioned at an average distance between the lens and the film surface rather than at the actual distance between the film surface and the optics which may vary due to film surface irregularities and the like. Since the magnification factor of the optics is usually several hundred times, even slight film surface irregularities may prevent accurate focusing or, alternatively, require a re-setting of the optics each time the microfiche film is newly indexed. This is time consuming and inconvenient.
In spite of such shortcomings, microfiche readers continue to be constructed along these lines. It appears that in the past it was thought necessary or at least desirable to maintain the film taut and flat by compressing it with a glass plate. Furthermore, such glass plates provided convenient means for spacing the optics from the film and fairly accurately controlling such spacing although, as pointed out above, precise spacing is not possible.